Acrylonitrile-butadiene-styrene copolymer resin (hereinafter, ABS resin) generally has a good balance of physical properties such as processability of styrene, chemical resistance of acrylonitrile, and flexibility and impact resistance of butadiene, and has an excellent appearance. Therefore, ABS resins have been widely used in automobile parts, electronic articles, sheet belts and the like. However, ABS resins are typically opaque, which limits their use in many applications.
ABS resins are opaque because the refractive index of the matrix resin is different from the refractive index of the dispersed rubber phase. Accordingly, light is refracted at the interface therebetween and light in the visible wavelength region is diffused depending upon the size of the rubber particles.
Generally, polystyrene resin has a high refractive index. In contrast, the dispersed phase (rubber) has a low refractive index due to the butadiene component. Therefore, for the ABS resin to be transparent, the refractive index of the rubber phase should be identical to that of the continuous phase (matrix resin). Further, for the ABS resin to have a good balance of flowability, impact resistance, gloss and transparency, the size of the rubber particles should be adjusted properly and the particle size distribution should be uniform to minimize the diffusion of the light in the visible wavelength region.
Rubber-modified styrenic resin is typically prepared by emulsion polymerization, suspension polymerization, solution polymerization, bulk polymerization, and the like. A disadvantage of emulsion polymerization is that residual emulsifier, electrolyte and coagulating agent can degrade the physical properties of the final products. In suspension polymerization, it can be difficult to control the rubber particle size and to remove an inorganic suspending agent employed during suspension polymerization. For this reason, organic suspending agents have been recently used. However, these organic suspending agents may also remain in the final product, resulting in the decrease of physical properties of the product. Furthermore, emulsion polymerization and suspension polymerization require separate dehydration and drying steps after polymerization, because they employ water as a polymerization medium.
Solution polymerization or bulk polymerization may provide a high-purity product, since they do not require emulsifiers or suspending agents typically employed in emulsion polymerization or suspension polymerization. Further, solution and bulk polymerization have the advantage of low production cost and large-scale production. They are, however, disadvantageous for preparation of transparent rubber-modified styrenic resin, because it is difficult to match the refractive indices between the dispersed phase (rubber) and the continuous phase (matrix resin).
Japanese Patent Laid-open Publication No. 2001-31833 discloses a transparent thermoplastic resin composition produced by adding a specific graft copolymer which contains a rubber component to a resin which does not contain a rubber component melted during continuous bulk polymerization followed by mixing the blend to adjust refractive index. The rubber and resin of the resultant product may have similar refractive index values and the product may have increased mechanical properties. The process, however, is not economically feasible because it requires an additional compounding process and the transparency of the resin composition obtained therefrom may be degraded.
U.S. Publication No. 2002/0032282 discloses a transparent rubber-modified styrenic resin composition with improved chemical resistance and moldability prepared by controlling the specific morphology of rubber particles and specific molecular distribution of the matrix resin in a continuous bulk polymerization to disperse the rubber particles (dispersed phase) into the matrix resin (continuous phase). This resin may have a good balance of flowability and impact resistance; however, the resin also has low transparency because the refractive index is not controlled during the reactions.
In general, conversion rates of monomers even under the same polymerization conditions are different from each other, thereby reducing transparency due to the difference in refractive index.
Accordingly, the present inventors have developed a transparent rubber-modified styrenic resin having excellent flowability, transparency and gloss while maintaining good mechanical properties and a method for continuous bulk polymerization thereof by adjusting the difference of refractive index between a rubber phase (styrene-butadiene rubbery copolymer) and the matrix resin (methylmethacrylate-styrene-acrylonitrile terpolymer) to be about 0.005 or less at each polymerization step.